Ecological and human health risk assessment of heavy metal(loid)s in agricultural soil in hotbed chives hometown of Tangchang, Southwest China

To determine the heavy metal(loid)s (HMs) contamination of agricultural soil in hotbed chives hometown of Tangchang, 788 topsoil samples were collected and analyzed for their heavy metal(loid)s concentration. The index of geo-accumulation (Igeo), pollution index (PI) and potential ecological risk index (EIi) were used to assess the degree of pollution. Correlation analysis and principal component analysis (PCA) were used to determine the sources of soil HMs. Human health risks estimated with hazard index (HI) and carcinogenic risk (CR) indices based on ingestion, inhalation and dermal exposure pathways for adults and children. The mean values of Cd, Hg, As, Pb, Cr, Cu, Ni and Zn were 0.221, 0.155, 9.76, 32.2, 91.9, 35.2, 37.1 and 108.8 mg kg−1, respectively, which did not exceed the threshold values of the risk screening value for soil contamination. The potential ecological risk of soil heavy metal(loid)s was low level and there was no significant human health risk. Based on PCA, Pb and Hg may originate from transportation and atmospheric deposition, Zn, Cr and Ni may originate from natural sources and industrial activities, and Cu and Cd may originate from agricultural activities. Overall, from the perspective of HMs content, the soil quality in this study area was at a clean level. This study provides a reference and a basis for formulating effective measures to prevent and control HMs enrichment in agricultural soils.


Scientific Reports
| (2022) 12:8563 | https://doi.org/10.1038/s41598-022-11397-0 www.nature.com/scientificreports/ Sample collection and measurement. Field sampling was performed in April 2021. The sample collection was done based on specification of land quality geochemical assessment (DZ/T 0295-2016), a total of 788 topsoil (0-20 cm) samples were collected. The sampling locations are shown in Fig. 1. At the same time, to improve the representativeness of the soil samples, five sub-samples were collected from a 20-50 m region around each sampling point by X-type sampling method and mixed into one sample, the sampling sites were located using a portable GPS. The samples were stored in sealed polyethylene bags until they were transported to the laboratory and were naturally air-dried for one week, removed other debris, and then sieved through a 10-mesh plastic sieve for later use.
In order to determine heavy metals contents of Cu, Pb, Zn, Cr, Ni and Cd, 0.1000 g soil samples were weighed accurately, placed into a PVC digestion vessel, and then digested with 10 ml HNO 3 -HCl-HClO 4 -HF (excellent grade). The concentrations of Cu, Pb, Zn, Ni and Cd were measured by inductively coupled plasma mass spectrometry (ICP-MS, NexION350X, USA) 19 , Cr were measured by inductively coupled plasma atomic emission spectrometry (ICP-AES, Avio500, USA). For As and Hg, 0.2500 g soil samples were weighed, placed into a tall beaker, and by water bath digestion with 10 ml aqua regia, the contents were measured by atomic fluorescence spectrophotometry (AFS, AFS8500, CHN) 20,21 . Soil pH was measured by ion selective electrode method 22 , the content of total potassium (TK) and total phosphorus (TP) was measured by X-ray fluorescence (XRF, Axios mAX, NLD) 23 , soil total nitrogen (TN) was measured by elemental analyzer method 24 , and the content of organic matter (OM) was measured by volumetric method 25 . In the process of measurement, duplicate tests were carried out, and quality control procedures were conducted using state first-level standard materials (GSS5, 7-9, 20, 23, 24, 28). The Primary original qualified rate of all state first-level standard materials not lesser than 98%. The limit of detection (LOD) values for Cu, Pb, Zn, Ni, Cd, Cr, As and Hg were obtained to be 1, 2, 4, 2, 0.01, 5, 0.5 μg g −1 and 0.0003 μg g −1 , respectively. There were 64 samples for duplicate tests, the qualified rate of duplicate tests for HMs were 100%, except Hg was 93.8%.

Index of geo-accumulation.
To assess the degree of contamination and compare concentration of different HMs in soil samples, the index of geo-accumulation (I geo ) was used, defined as follows 2 : where C i is the concentration of i element at the sampling site (mg kg −1 ); B i is the geochemical background concentration (mg kg −1 ) of i element in Chengdu 26 , K is a constant value and equals to 1.5. There are seven classes based on I geo value: ≥ 5, 4-5, 3-4, 2-3, 1-2, 0-1, < 0, representing extremely contaminated, strongly to extremely contaminated, strongly contaminated, moderately to strongly contaminated, moderately contaminated, uncontaminated to moderately contaminated, uncontaminated, respectively.
Pollution index and synthetic pollution index. In order to assess the level of HMs pollution in the soil, a single factor pollution index (PI) and a synthetic pollution index (SPI) were calculated: where PI is the pollution index of each element and SPI is the synthetical score of each heavy metal(loid)s to the composite pollution. S i is the evaluation standard of the i element, and the national control thresholds were chosen as the standard (Table 1). elements. There are five pollution categories based on PI and SPI values: < 0.7, 0.7-1, 1-2, 2-3, ≥ 3, representing safety, alert, low pollution, moderate pollution, and severe pollution, respectively 27 .
Potential ecological risk assessment. The potential ecological risk index (EI i ) and comprehensive potential ecological risk index (RI) were proposed by Lars 28 . Which were employed to assess the degree of ecological risk based on the toxicity, concentrations, characteristics and environmental behavior of HMs.
where EI i is the potential ecological risk factor of individual HMs, T i is the toxic response factor of single HMs. In this study, the T i values of Hg, Cd, As, Cu, Ni, Pb, Cr, and Zn were 40, 30, 10, 5, 5, 5, 2, and 1, respectively 29 . The EI i is defined in five categories as: low (< 40), moderate , considerable (80-160), high (160-320) and very high (≥ 320). Five categories are defined for RI: low (< 150), moderate (150-300), considerable (300-600), very high (600-1200), and dangerous (≥ 1200), representing 30 . www.nature.com/scientificreports/ Health risk assessment. The human health risk assessment of HMs in soils is widely carried out using exposure evaluation. In this study, three exposure pathways associated with soil HMs were explored: ingestion (ADD ing ), inhalation (ADD inh ) and dermal contact (ADD derm ). The estimated average daily intake of HMs (ADI, mg kg −1 day −1 ) via ADD ing , ADD inh and ADD derm for both adults and children was as follows 31,32 .
Detailed parameter values of IngR, InhR, EF, ED, BW, AT, PEF, SA, AF and ABS are listed in Table 2.  Table 2. Exposure parameters, reference dose (RfD) and slope factor (SF) of HMs.

Parameters Value
IngR (ingestion rate of soil) (mg day www.nature.com/scientificreports/ Hazard index (HI, unitless) and hazard quotient (HQ, unitless) are usually used to indicate the risk level of human exposure for non-carcinogenic contaminants. Carcinogenic risk (CR, unitless) is usually used to indicate the probability of inducing cancer when humans are exposed for carcinogenic contaminants. When the contaminants enter through multiple pathways of exposure, assuming that there is no antagonistic effect or cooperative effect between the contaminants, the HI for all exposure pathways and the CR can be calculated as follows 1 : where SF is the cancer slope factor of the HMs via ADD ing , ADD inh and ADD derm of soil particles (mg kg −1 day −1 ) and RfD is the reference dose of the HMs through ADD ing , ADD inh and ADD derm of soil particles (mg kg −1 day −1 ). Detailed values are listed in Table 2. Generally, HQ or HI is ≤ 1, it is considered that there are no significant risks of noncarcinogenic effects. For CR, the acceptable risk for regulatory purposes is in the range from 1.0 × 10 −6 to 1.0 × 10 −4 .

Data analysis.
Descriptive statistics, such as mean, standard deviation, median, maximum, minimum and coefficient of variation were used to characterize the contents of HMs in soil samples. Microsoft Excel 2010 and SPSS 26 and Origin 2019b were used to process the experimental data, perform for correlation analysis of HMs concentrations, and test significance analysis for soils properties and biomarker responses at P < 0.05 and P < 0.01 levels. Adobe Origin 2019b was also taken to draw graphics.

Results and discussion
Soil physical-chemical properties and HMs concentrations. The soil physical-chemical properties and HMs concentrations are summarized in Table 1. The soil mean pH value was 6.17 and ranged from 4.16 to 9.04 in different sites. The samples sites of level for pH ≤ 6.0(acidic soil), 6.0 < pH ≤ 7.5(Neutral soil) and pH > 7.5(alkaline soil) were 51.5%, 36.4% and 12.1%, respectively. The average content of TN, TP and TK were 1.33 g kg −1 , 1.16 g kg −1 and 23.6 g kg −1 , and ranged from 0.7 g kg −1 to 2.4 g kg −1 , 0.22 g kg −1 to 20.8 g kg −1 and 10.3 g k g −1 to 28 g kg −1 , respectively.
The  , indicating that the soil HMs Zn, Cu, Cr, Ni, Pb, As and Cd in the study area were generally in a no contamination according to the defined classes, while Hg was in uncontaminated to moderately contaminated. The I geo values for Zn, Cu, Cr, Ni, Pb, As and Cd in more than 90% of samples were less than zero, only a few outliers in the soil were classified as moderately contaminated or worse. Which meaning point pollution at those sample sites. However, the I geo values for Hg in 47.34%, 40.61%,10.66% and 1.40% of samples were belonged to < 0, 0-1, 1-2 and 2-3, respectively, 52.66% sample sites were contaminated moderate to strongly. It means that there were non-point source pollution sources of Hg in the study area. Relevant studies have pointed out that the overall distribution trend of soil Hg content in Chengdu Plain is relatively high in the north, which is mainly affected by geological structure, domestic pollution of urban residents and pollutant emission of industrial enterprises 17 .
Pollution index. The PI and SPI of soil HMs are drawn in Fig. 2. The PI average value of Cd, Hg, As, Pb, Cr, Cu, Ni and Zn were 0.69, 0.09, 0.28, 0.36, 0.56, 0.59, 0.49 and 0.50, and the ranging from 0.17 to 1.60, 0.01 to 0.68, 0.12 to 0.62, 0.12 to 1.29, 0.25 to 1.06, 0.19 to 6.07, 0.14 to 0.72 and 0.18 to 7.28, respectively. The PI value for Cd in 55 sample sites, for Cu in 5 sample sites, and for Pb, Cr and Zn in one site were belong to 1.0-2.0, indicating low pollution according to the defined classes. However PI value for Cu(6.07) and Zn(7.28) in one site were higher 3, belong to the severe pollution. It may be because the sampling point was close to the industrial area. Although the I geo mean value of Hg was the largest, its PI average value was the lowest. Because of different research perspectives and different reference indicators, the results were inconsistent 33 .
The SPI is usually applied to evaluate the overall status of HMs contamination. The mean SPI value was 0.62, ranging from 0.31 to 5.22 in Tangchang agricultural soil. In our study, 98.23% of soil samples were not polluted with HMs as 1.52% of samples within low pollution and 0.25% of the samples had a severe pollution level. In general, the soil quality in the study area is generally at a clean level.
Potential ecological risk assessment. The EI i values for each HMs and the RI are shown in Fig. 2. The EI i values of As, Pb, Cr, Ni and Zn in all sample sites were less than 40, meaning a low potential ecological risk with these HMs. For Cu, except that the EI i in one sample was 108, which belongs to considerable potential ecological risk, the other samples were less than 40. For Cd and Hg, 93.4% and 9.5% of soil samples were in low potential ecological risk category, respectively. Significantly, 6.5% of samples for Cd and 55.5% for Hg were in moderate potential ecological risk categories, and one sample for Cd and 29.4% for Hg were in very high potential ecological risk categories. Especially EI i of two soil samples for Hg were high than 320, belongs in dangerous potential ecological risk categories. In this study area the RI mean value was 135.4, and ranging from 62.6 to 514.6. In all samples 76.4% had a low potential ecological risk of HMs, 22.1% had a moderate risk and 1.5% had a considerable risk.
According to the calculation results of I geo and EI i , it is found that the results of the two methods were consistent and different. Such as the I geo and EI i of Hg were the largest, however, the I geo of Cd is the smallest, but its EI i was the second highest. The reason may be that the I geo focuses on the enrichment of exogenous HMs, on this basis, the EI i focuses more on the potential effects of toxic effects of HMs. In the EI i , the toxicity coefficients of Hg and Cd were the largest, which were 40 and 30 respectively. The reference ratios of the two methods were the same soil background value, so the difference of toxicity coefficients leads to great changes in EI i . Compared with I geo , the EI i considers not only the content of HMs, but also the biological toxicity of different metals 34,35 . Sources of HMs in agricultural soils. Correlation analysis. Correlation tests were used to understand the relationship between different HMs and find their possible sources 36 . If there is a positive correlation between HMs may be indicative of their common source. As shown in Table 3, the most of HMs exhibited significant correlations(P < 0.05). Especially, Hg-Cd, As-Cd, Cr-Pb, Cu-Cd, Ni-Hg-Pb, Ni-Cu and Zn-Cr-Ni exhibited high significant correlations(P < 0.01), which meaning that these HMs in the study area may have common origin. Moreover, significant correlations were also observed between pH with Cd, Pb, Ni and Cr, TN with Cd, Hg, As and Pb, TP with Cd, As and Cu, TK with Hg, As, Pb, Cr and Ni, OM with Cd, Cr, Hg, As, Pb and Cu, which indicate that the source of HMs in the study area may be closely related to human activities.  (Table 1). High value points for Pb and Hg were mainly concentrated in TangChang town government and surrounding residential areas. It seems that anthropogenic activities sources such as traffic and atmospheric subsidence were the major sources of Pb and Hg in agricultural soils. PC-2 consists of Zn, Cr and Ni, accounted for 20.446% of the total variance. These results were consistent with the results of Pearson's correlation analysis (Table 3). Although many researchers have found that these elements had some degree of homology and may be affected mainly by material and pedogenic processes 30,31,37 , but their mean concentrations higher than the soil background value for Chengdu. Therefore, the natural sources and industrial activities together constituted the major sources of PC-2. PC-3 consists of Cu and Cd, accounted for 17.063% of the total variance. Very significant correlation was found with Cu-P and Cd-P. The planting of hotbed chives needs to use a lot of phosphate fertilizer. Therefore, it seems that long-term use of chemical fertilizers and pesticides was the www.nature.com/scientificreports/ major source of PC-3. PC-4 includes only As, accounted for 15.271% of the total variance. The As in the soil of the whole town was generally in a pollution-free state. It seems that the natural sources were the major sources of PC-4 and many studies have reached similar conclusions 38,39 .
Health risk assessment. Non-carcinogenic risk assessment. Table 5 and Fig. 3 show the results for noncarcinogenic risk. In collected soil samples the HQ value of three exposure pathways (ingestion, dermal contact, and inhalation) for eight HMs were lower than 1. The HI for adults and children were 0.173 and 0.996, respectively. Suggesting no significant non-carcinogenic risk in the study soils. It is noteworthy that the HI of children Table 3. Results of Pearson's correlation analysis of HMs. *Shows significant correlation at the 0.05 level (2-tailed). **Shows significant correlation at the 0.01 level (2-tailed).    Fig. 3A,B, It was easy to show that the contribution of different pathways to non-carcinogenic risk was similar between adults and children decreased in the following order: Ingestion > Dermal contact > Inhalation. The HI mean values for adults and children by ingestion, dermal contact and inhalation routes were 0.129 and 0.919, 0.0434 and 0.0761, 0.000757 and 0.0014, respectively. It's not hard to saw the HI for ingestion route of both adults and children were 1-3 orders of magnitude higher than the other two exposure pathways. Many studies have reached similar conclusions 32,36 . Thus, the ingestion route may be an important pathway for HMs exposure in study area.
The HI mean value of single HM for children and adults decreased as following order (Fig. 3C,D): Cr > As > Pb > Ni > Cu > Hg > Zn > Cd. Cr, As and Pb were the largest contributors for both adults and children, accounting for 47.33% and 42.37%, 32.68% and 39.64%, 15.95% and 13.26%, respectively. Indicating that attention should be pain to the Cr, As and Pb elements due to their noncarcinogenic risk. Which was basically consistent with the research results of Bo et al. 1 and Bao et al. 32 . Overall, The HMs of Cr, As and Pb were the main non-carcinogenic factors in soil in the study area, and the risk control of these elements should be strengthened.
Carcinogenic risk assessment. The CR of the HMs are shown in Table 5. Three exposure pathways were considered for Cd and Hg in our study. But As and Pb were considered carcinogenic by ingestion and inhalation, and Cr was considered carcinogenic through inhalation. The TCR mean values were 3.68 × 10 −5 for adults and 6.27 × 10 −5 for children, obviously were in the range 1 × 10 −4 from 1 × 10 −6 , suggesting the TCR caused by HMs in the study area was acceptable on the whole, but it still exceeds the soil treatment threshold value 10 −6 . The CR average values through the three exposure pathways were CR ing 3.65 × 10 −5 , CR inh 2.53 × 10 −7 and CR derm 1.07 × 10 −7 for adults, CR ing 6.25 × 10 −5 , CR inh 1.12 × 10 −7 and CR derm 4.50 × 10 −8 for adults. Clearly the CR ing was much larger than CR inh and CR derm for both adults and children, which indicates that oral ingestion is the major exposure pathway for CR. Which was consistent with the research results of Song et al. 31 and Bo et al. 1 . For single HM, the CR value of Pb and Hg for adults were 2.72 × 10 −5 and 8.6 × 10 −6 , and Pb, Hg and Cd for children were 4.63 × 10 −5 , 1.48 × 10 −5 and 1.36 × 10 −6 , respectively, within acceptable criterion. Overall, the longterm health effects for adults and children are not serious at current single HM level.
In this study, the total contents of HMs in the soils were used to assess health risk, the bioavailability of HMs were not considered, which may have caused the assessment results to be higher than the actual local situation 1,31,42 . In addition, because the parameters of health risk evaluation for children were set to be more sensitive than those for adults, the non-carcinogenic risk and carcinogenic risk for children were higher than those for adults 2,43,44 . However, those risks were at acceptable or negligible levels. Therefore, the study area is suitable for safe and clean production of hotbed chives.

Conclusion
In the study area, the average contents of HMs Cd, Hg, Ni, As, Cu, Cr, Pb and Zn in the soil do not exceed the threshold values of the risk screening values for soil contamination, and are generally at a clean level. In addition to natural sources, Pb and Hg may mainly come from transportation and atmospheric deposition, Zn, Cr and Ni may mainly come from industrial activities, and Cu and Cd may mainly come from agricultural activities such as fertilization, pesticide application and irrigation. In general, the potential ecological risk of soil HMs is low, but a few areas had a moderate risk or considerable risk. Overall, the HMs of Cr, As and Pb are the main non-carcinogenic factors in soil in study area. Although there is no significant human health risk, however, the non-carcinogenic risk and carcinogenic risk for children are higher than those for adults. In order to reduce the content and human health risks of heavy metal(loid)s in agricultural soils, ensure the green and sustainable production capacity of the soil in hotbed chives production area, strengthening soil management, reducing